The first commercially attractive cellular telephones or terminals were introduced in the market at the end of the 1980's. Since then, a lot of effort has been made in making smaller terminals, with much help from the miniaturisation of electronic components and the development of more efficient batteries. Today, numerous manufacturers offer pocket-sized terminals with a wide variety of capabilities and services, such as packet-oriented transmission and multiple radio band coverage.
In order to attract customers the terminal manufacturers have therefore taken further measures to strengthen their position in the competition, one such being to offer different types of accessories for the terminals. Such accessories include auxiliary keyboards, media players, headsets and battery chargers. Each of these accessories may be connectable to separate sockets of the terminal, provided for each particular purpose. However, in many cases the terminal includes a system connector to which several different types of accessories may be connected. For this reason, the terminal needs to be capable of identifying which type of accessory is connected.
In a prior art solution, applied in the Ericsson® T28 terminal series, the 3 volt system connector made use of break signals for data communication, and portable hands free sense, as the only passive accessory detection possibility.